Snake venom is a mixture of proteins, toxic and nontoxic components. It has been known that venoms of some Crotalidae and Viperidae have anticoagulation activity. Recent work in our laboratory and also in Dr. Siigur's laboratory isolated fibrinolytic enzymes without toxic action from the venoms of Vipera lebetina (lebetase) and Crotalus Atrox (atroxase). Thrombosis is a leading killer throughout the world, and it is important to find better thrombolytic agents than those currently used such as tpA, urokinase, and streptokinase. Lebetase, atroxase, and other fibrinolytic enzymes from snake venoms are potentially useful thrombolytic agents after the removal of toxic components from snake venoms. It is proposed that base sequence of cDNA encoded for lebetase will be determined. For this purpose mRNA will be first isolated from venom glands of Vipera lebetina. Lebetase gene will be isolated and sequenced. Recombinant lebetase will also be produced by packaging lebetase gene into baculovirus transfer vector. The cloned lebetase will be compared with the enzyme isolated from Vipera lebetina venom. Once cloned lebetase is shown to be identical to the natural one, fibrinogenase activity can be increased by modifying the enzyme in the future. Our proposed work may produce a new potent thrombolytic agent.